1. Field of the Invention
The present invention relates to an oscillator, and more particularly to an oscillator which can be used as a clock generator for use in a microcomputer.
2. Description of related art
Heretofore, microcomputers are usably incorporated with a clock oscillator of the type which includes a NOT circuit having an output thereof fed back to an input thereof through a frequency control circuit having a quartz-crystal resonator or a ceramic resonator, so that a signal of the frequency determined by the frequency control circuit is generated. This type of oscillator is very excellent in precision of oscillation frequency, but is not so good in starting characteristics. Briefly, a relatively long time is needed from the power-on to stabilization of the oscillation frequency. For example, if a ceramic resonator is used, it takes several milliseconds to several ten milliseconds for stabilization of frequency. In addition, during the start period, oscillation will occur at a frequency higher than a stabilized frequency.
If the above mentioned oscillator is used as a clock generator for a microcomputer, during a short time period just after the power-on, since a clock frequency will be higher than a stabilized clock frequency, the microcomputer will often malfunction. For example, error will occur in reading data from a read only memory. To avoid this problem, so-called "power-on-clear" method is used in which a microcomputer is reset when a predetermined period of time has elapsed from the power-on. However, this method is not suitable to a remote controlled computer which is demanded to minimize the consumed power. The remote controlled computer is ordinarily constructed such that a clock generator oscillates only when it receives an instruction from an external device, and a power supply for the microcomputer is turned on in response to the oscillation signal to cause the microcomputer to operate in synchronism with the oscillation clock, and then, the power supply is automatically turned off when the microcomputer has completed a required processing. Therefore, the power consumed from the power-on to the so-called "power-on-clear" is not negligible in minimizing the power consumption.